This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-71572 filed on Mar. 15, 2002.
The present invention relates to a capacitive device that includes a fixed electrode and a movable electrode, which moves in response to an inertial force and an electrostatic force that act on the movable electrode.
Research and development work is underway on a sensor that is capable of measuring a physical quantity such as acceleration and angular velocity that is related to an inertial force along directions substantially orthogonal to a surface of the substrate of the sensor. The sensor includes a movable electrode and a fixed electrode. The movable electrode floats above the substrate surface and can move with respect to the substrate along directions substantially orthogonal to the substrate surface. The fixed electrode is fixed to the substrate and stationary with respect to the substrate.
Such a sensor is under development in order to meet a need to measure not only a physical quantity substantially parallel to the substrate surface but also a physical quantity substantially orthogonal to the substrate surface, as well as a need to place the substrate surface substantially orthogonally to the directions along which the inertial force related to the physical quantity is applied for the sake of stability. With respect to such a sensor, there is a desire not only to measure the magnitude of the physical quantity but also to detect in which one of the directions that are substantially orthogonal to the substrate surface the inertial force acts.
A first proposed sensor for the desire includes a substrate, a plate-like weight that is parallel to a surface of the substrate and movable orthogonally to the surface of the substrate, a movable electrode that is located on a surface of the weight, and a fixed electrode placed above the weight to face the movable electrode. When the movable electrode is displaced with the weight toward the fixed electrode, or in a first direction that is substantially orthogonal to the substrate surface, a distance between the movable electrode and the fixed electrode decreases, and consequently, the capacitance formed therebetween increases.
When the weight is displaced in the direction away from the fixed electrode, or in a second direction that are substantially opposite to the first direction, the distance between the movable electrode and the fixed electrode increases, and, as a result, the capacitance formed therebetween decreases. Therefore, it is possible to detect in which one of the first and second directions the inertial force acts on the basis of whether the capacitance is increasing or decreasing, even if the variance of the capacitance is the same.
In the first proposed sensor, however, the movable electrode and the fixed electrode are stacked to face each other along the directions orthogonal to the substrate surface. In order to realize such a structure, a layer corresponding to the movable electrode, a layer corresponding to the fixed electrode, and a sacrificial layer need to be stacked first such that the sacrificial layer becomes located between the other two layers, and then the sacrificial layer needs to be stripped off by a complex etching process.
Alternatively, a publication JP-A-2000-49358 discloses a sensor (second proposed sensor) that is capable of detecting the direction along which the inertial force acts, even though the fixed electrode and the movable electrode of the sensor are formed from a single layer.
As shown in FIG. 1, the second proposed sensor 1 includes a movable electrode 5, which floats above a surface of a substrate 2 and is movable along the directions that are substantially orthogonal to the substrate, or along the z-axis of FIG. 1, and a fixed electrode 6, which is fixed onto the substrate 2. The second proposed sensor 1 measures a physical quantity that is related to an inertial force applied on the sensor 1 on the basis of the variance in the capacitance between the movable electrode 5 and the fixed electrode 6 when the movable electrode 5 moves in response to the inertial force along the directions that are substantially orthogonal to the substrate surface.
For example, when the movable electrode 5 moves in the positive direction along the z-axis as shown in FIG. 2, which is a first direction that is substantially orthogonal to the substrate surface, the capacitance increases because the overlap between the movable electrode 5 and the fixed electrode 6 increases to S10 in comparison with the overlap in FIG. 1. On the other hand, when the movable electrode 5 moves in a negative direction along the z-axis as shown in FIG. 3, which is a second direction that is substantially orthogonal to the substrate surface, the overlap becomes smaller to S20 in comparison with the overlap in FIG. 1, and, consequently, the capacitance decreases. As a result, it is possible to detect in which one of the first and second directions the inertial force acts on the basis of whether the capacitance is increasing or decreasing, even if the variance of the capacitance is the same. The second proposed sensor 1 of FIG. 1, however, includes a space 8, where a sacrificial layer having steps was located in the manufacturing process of the second proposed sensor 1. Therefore, a complicated manufacturing process is required in order to form the sacrificial layer.
Alternatively, a sensor (third proposed sensor) is proposed in J. H. Daniel, D. F. Moore, Sensors and Actuators A73 (1999), pages 201-209. In the third proposed sensor, the electrode-confronting surfaces of the fixed and movable electrodes of the sensor, at which the fixed and movable electrodes face each other, are tilted at an angle with respect to the directions that are substantially orthogonal to a substrate surface of the third proposed sensor, so that the capacitance between the electrodes increases when the movable electrode moves in a first direction that is substantially orthogonal to the substrate surface, while the capacitance decreases when the movable electrode moves in a second direction that is substantially opposite to the first direction. In the third proposed sensor, however, an advanced manufacturing process is required for creating the structure in which the electrode-confronting surfaces of the fixed and movable electrodes are tilted at an angle with respect to the directions that are substantially orthogonal to the substrate surface.
In the second proposed sensor 1, if the surfaces of the electrodes at which the electrodes face the substrate surface might be substantially planar and parallel to the substrate surface and if the surfaces of the electrodes might be in approximately the same distance from the substrate surface before the movable electrode is dislocated, then the sacrificial layer to form the part 8 would not need the steps and it would be possible to manufacture the sensor 1 using a relatively simple manufacturing process. In the third proposed sensor as well, if the electrode-confronting surfaces of the electrodes would not need to be tilted at an angle with respect to the directions that are substantially orthogonal to the substrate surface and the electrode-confronting surfaces might be orthogonal to the substrate surface, it would be possible to manufacture the third proposed sensor using a relatively simple manufacturing process.
When the above structures were to be adopted in the proposed sensors, however, it would be difficult to increase the area-distance quotient, which is obtained by dividing the overlapping area between the electrodes by the distance between the electrodes. The capacitance between the electrodes substantially varies in proportion to the area-distance quotient, and it would basically only be possible to decrease or keep the area-distance quotient, no matter in which direction the movable electrode moves along the directions that are substantially orthogonal to the substrate surface if the above structures were to be adopted. Therefore, it would be difficult to make the capacitance greater than that when the movable electrode at the initial position if the above structures were to be adopted.
For this reason, it had been considered difficult to realize a physical quantity sensor that is capable of detecting the direction along which the inertial force acts on the sensor using the above structures, which can simplify the manufacturing process of the sensor.
The present invention has been made in view of the above aspects. A first object of the present invention is to simplify the structure of a capacitive physical quantity sensor that is capable of detecting the direction along which an inertial force acts on the sensor in order to simplify the manufacturing process of the sensor. A second object of the present invention is to provide other types of capacitive devices that include the same electrode structure that is used in the capacitive physical quantity sensor.
To achieve the above objects, a capacitive device according to the present invention includes a substrate, a movable electrode, and a fixed electrode. The movable electrode is located above a surface of the substrate and is movable with respect to the substrate along directions that are substantially orthogonal to the surface. The movable electrode includes a substrate-confronting surface, at which the movable electrode confronts the surface of the substrate, and an electrode-confronting surface.
The fixed electrode is stationary with respect to the substrate. The fixed electrode includes a substrate-confronting surface, at which the fixed electrode confronts the surface of the substrate, and an electrode-confronting surface. The substrate-confronting surfaces are substantially parallel to the surface of the substrate. The substrate-confronting surfaces are substantially planar and substantially at the same level along the directions that are substantially orthogonal to the surface of the substrate before the movable electrode is displaced.
The electrode-confronting surfaces confront each other and are substantially orthogonal to the surface of the substrate. When the movable electrode is displaced in a first direction that is substantially orthogonal to the surface of the substrate, the total sum of area-distance quotients in the overlap between the electrode-confronting surfaces remains substantially unchanged or decreases to provide a first reduction rate that is substantially zero or more. On the other hand, when the movable electrode is displaced in a second direction that is substantially opposite to the first direction, the total sum of area-distance quotients remains substantially unchanged or decreases to provide a second reduction rate that is substantially zero or more. The reduction rates are different from each other.
In the capacitive device according to the present invention, fringe capacitances are created between the edges of the electrode-confronting surfaces of the movable electrode and the fixed electrode. Due to the fringe capacitances, the net capacitance between the electrodes increases when the movable electrode is displaced in one of the first and second directions, the reduction rate in which is smaller than the other. On the other hand, even though the fringe capacitances are added, the net capacitance decreases when the movable electrode is displaced in the other of the directions. As a result, it is possible to detect in which direction along the directions that are substantially orthogonal to the surface of the substrate the movable electrode is displaced on the basis of the increase or the decrease in the net capacitance.
In the capacitive device according to the present invention, the substrate-confronting surfaces are substantially parallel to the surface of the substrate. In addition, the substrate-confronting surfaces are substantially planar and substantially at the same level along the directions that are substantially orthogonal to the surface of the substrate before the movable electrode is displaced. As a result, the sacrificial layer used for manufacturing the capacitive device according to the present invention does not need such a complex manufacturing process that is used for forming the sacrificial layer having the steps of the second proposed sensor.
Furthermore, in the capacitive device according to the present invention, the electrode-confronting surfaces confront each other and are substantially orthogonal to the surface of the substrate. Therefore, the advanced manufacturing process for forming the tilted electrode-confronting surfaces of the third proposed sensor is not necessary. Thus, the manufacturing process of the capacitive device according to the present invention is relatively short and simple.
With the electrode structure of the capacitive device according to the present invention, when a potential difference is applied between the movable electrode and the fixed electrode, the electrodes are electrostatically attracted to each other. In addition, the electrostatic attraction that acts on the movable electrode forces the movable electrode to move in one of the first and second directions. In other word, it is possible to arbitrarily drive the movable electrode along the directions that are substantially orthogonal to the surface of the substrate using the potential difference. Thus, a variety of capacitive devices that make use of the drivability are achieved according to the present invention.